1. Field of the Invention
This invention relates to an improved gas-liquid contacting tray of the type used in the distillation and and absorption systems for mass transfer between two fluids.
2. Description of the Prior Art
In the art of mass transfer for selectively separating at least one component from a mixture of at least two constituents as for example in distillation and absorption applications, an upwardly flowing vapor or gas stream is typically contacted on a substantially horizontally aligned contacting surface with a generally downwardly flowing liquid stream. In a conventional distillation process, such contacting permits the upwardly flowing vapor or gas stream to become selectively enriched with the lighter components of the mixture, i.e., those components with relatively high volatilities, while the generally downwardly flowing liquid stream becomes selectively enriched with the heavier components of relatively low volatilities.
Two general types of liquid-gas contacting trays are widely used in distillation and absorption applications, bubble cap trays and perforated trays. Due to the broad utility of perforated trays in low pressure separations as for example for the separation of thermally sensitive compounds, as a consequence of the low tray pressure drop characteristic of perforated trays relative to bubble cap trays, and because of their simple and relatively inexpensive design, perforated trays are widely employed and are displacing the use of bubble cap trays in many applications in which the latter were formerly employed.
Although numerous types of gas-liquid contacting trays can be groups under the general classification of perforated trays, including many proprietary valve type trays, the most common type in conventional use is the so-called sieve tray. This tray is typically constructed with a flat tray member perforated with a multiplicity of round holes. Such perforations provide passageways for the upflowing contacting vapor, which then intimately comingles with liquid flowing across the tray member. Successive trays within a liquid-gas contacting column are interconnected by means of downcomer liquid discharge devices in a manner well known to those skilled in the art.
A widely employed sieve tray is the so-called single-pass cross-flow tray. In this tray design liquid discharges from an imperforate receiving area near the tray's outer edge, flows across an inlet weir which forms a chord of the circle defined by the column diameter, and then flows across the entire active surface of the tray member in first a diverging and then a converging flow pattern. The liquid after flowing across the tray member discharges at a similar outer edge region diametrically opposed to the liquid inlet area, and then is transferred by downcomer to the next lower tray where it is introduced immediately below the liquid discharge of the preceding overlying tray. the liquid then flows across the active surface of the underlying tray member in the opposite direction with respect to that of the preceding tray, and so on down the column.
The single-pass cross-flow tray has achieved widespread usage because of its simple and relatively inexpensive construction while simultaneously allowing high utilization of the available column area for gas-liquid contact along with high overall contacting efficiency levels. With respect to the latter characteristic, two types of contacting efficiencies concern the designer of gas-liquid contacting trays, point efficiencies and tray or plate efficiencies. Point efficiencies on the tray member contacting surface are principally determined by the physical and thermodynamic properties of the fluid system involved in the contacting operation, as well as by the degree of localized intimacy of contact between the gas and liquid phases involved. Since the designer has relatively little independent control over these factors, the point efficiencies of a given system under normal operating conditions are more or less invariant. Tray or plate efficiencies, on the other hand, are related to the aforementioned point efficiencies by a phenomenon termed "flow path enhancement" and, theoretically, can be higher than the point efficiencies measured at various points on the tray member contacting surface. The factors which limit the actual plate efficiency of a contacting tray surface include diffusive backmixing and departures from bulk plug flow across the tray surface. Although backmixing is a function of fluid properties and, therefore, is not under the designer's control, it is possible to control the flow distribution of the fluid on the tray member contacting surface. In fact, by providing an ideal plug flow of liquid across large diameter sieve trays, plate efficiencies higher than 100% are theoretically possible.
In spite of the foregoing considerations, it is well established in the separation art that large diameter sieve trays do not operate in an ideal fashion. In the first place, such trays typically require a large hydraulic gradient to promote liquid flow from the inlet to the outlet of the tray member. The requisite tray inlet liquid head results in considerable weeping in the inlet region of the tray. Such weeping is highly detrimental to overall contacting system efficiency because the liquid being processed effectively bypasses gas-liquid contact on two successive trays in the contacting column, bypassing from the inlet region of a first tray to the outlet region of a second tray and thence to the inlet area of a third tray.
U.S. Pat. No. 3,417,975 to B. Williams and E.F. Yendall describes a gas-liquid contact tray employing a uniform pattern of fixed size openings with walls normal to the tray surface and a uniform pattern of obliquely inclined openings, the latter being oriented in the downstream direction of liquid flow. Trays designed in accordance with William et al patent, hereafter referred to as slotted sieve trays, performed with improved efficiency owing to the elimination of longitudinal hydraulic gradient in the liquid on the tray. The additional degree of design freedom afforded by the two sets of fixed apertures results in utilization of only a predetermined, appropriate fraction of the total available vapor thrust to accomplish liquid transport across the tray. Other factors being equal, the neutralization of the hydraulic gradient now produces uniform resistance to vapor penetration through the liquid depth on the tray, and both the vapor and liquid flows are uniformly distributed over the active area of the tray. The propulsive effect of the vapor on the Williams et al tray balances the liquid hydraulic gradient thereon and accordingly eliminates the aforementioned weeping problem to a large extent.
In addition, conventional sieve trays are beset with a problem of inactivity which is manifested by the tendency of the unaerated liquid flowing on the tray from the liquid inlet to remain in an unaerated state on the tray member surface. Thus the liquid entering the tray often remains inactive for a substantial distance across the tray unless means is provided in its path to positively initiate bubbling activity. U.S. Pat. No. 3,282,576 to W. Bruckert describes a bubbling promotor, which when disposed at the tray inlet, increases momentarily the kinetic energy of the liquid -- hence, reducing its hydrostatic head. Bubbling is initiated immediately at the tray threshhold and once initiated, continues across the tray, thereby allowing full use of the contacting surface for gas-liquid mass transfer contacting.
The combination of these two technical innovations enables high localized point efficiencies to be achieved on the gas-liquid contacting surfaces of the sieve tray type. However, as is taught in U.S. Pat. No. 3,759,498 to L.C. Matsch, one additional tray feature is needed on large diameter trays with diverging-converging flow paths. To permit high overall trays efficiencies to be achieved on such trays, liquid flow must approximate the ideal plug flow profile as closely as possible. The Matsch patent teaches that the performance of single-pass cross-flow sieve trays can be significantly improved by utilizing a specific pattern of the vapor slot openings of the type as taught by the Williams et al patent. According to the Matsch teachings, the identification of certain key zones and the implementation of a certain slotting density and slotting orientation in each zone is necessary to eliminate problems resulting from unequal gas-liquid froth height and improper liquid distribution on the tray surface. More specifically, by progressively increasing the slot density in tray member regions downstream of the tray transverse center line and by progressively increasing the slotting angle relative to the tray's diametral streamline in regions located downstream of the transverse center line and transverse to the diametral stream line, the operation of a cross flow tray is considerably improved. As is implicit from the previous discussion, such improvement of tray performance arises from the flow path enhancement phenomenon since the liquid flow now approximates the ideal plug flow profile as it traverses the gas-liquid contacting surface of the tray.
For a given tray diameter, single-pass cross-flow trays have a limited liquid capacity, since the single liquid downcomer means tends to becomes overloaded at high liquid flow rates. At increasing liquid flow levels, the single downcomer of single-pass cross-flow trays will become increasingly filled with liquid, and as the liquid loading is further increased the liquid will eventually back-up onto the overlying tray surface. Such increased liquid level on the tray eventually chokes the tray resulting in massive entrainment. This condition, commonly referred to as "flooding", is accompanied by a sharp decline in tray efficiency and an increase in pressure drop across the tray. As a result, in those instances where high liquid loadings are necessary or desirable, more complex tray configurations employing different liquid flow patterns, such as two-pass cross-flow trays, may become more desirable. Such two-pass trays provide correspondingly more downcomer area and lower liquid loading per unit width of the active tray surface than an equivalent diameter single-pass cross-flow tray.
Under the foregoing considerations, any increase in the liquid handling capacity of liquid contacting trays which is derived from an increase in the number of downcomers or liquid inlet means is obtained at the expense of an increase in cost of the tray and a decrease in overall tray efficiency, for comparable localized point efficiencies on the tray, as a result of a decrease in the flow path enhancement phenomenon. In addition, in many instances an increase in the number of downcomers for the tray also results in a decrease in the total attainable active area of the tray. Accordingly, it becomes essential to maintain the flow profile as close as possible to ideal plug flow behavior on trays with differing flow patterns, so that overall tray efficiencies commensurate with single-path cross-flow trays can be maintained. Unfortunately, this has been difficult to achieve in practice, particularly with two-pass, side-to-center flow trays.
In the two-pass, side-to-center flow sieve tray, the liquid is introduced to the tray member contacting surface by a liquid inlet adjacent the outer periphery of the tray and flows in a diverging flow pattern to a liquid discharge outlet directly across the tray surface from the inlet, with the outlet extending transversely along a diameter of the tray. The problem of poor flow hydraulics on such two-pass, side-to-center flow trays is well known and documented in the art. Unfortunately, the prior art has not overcome the problem of correcting such hydraulic performance deficiency, which is the result of severe maldistribution of liquid on the tray member contacting surface. In the design of two-pass, side-to-center flow trays, the design approach for single-pass cross-flow trays has typically been applied, but such approach has proved unavailing in overcoming the liquid maldistribution problem on the tray. Nonetheless, relative to a single-pass tray for the same liquid flow, the liquid loading on a two-pass tray per unit tray width is only half as great. As a result of such difference, two-pass trays provide considerably higher liquid capacity and substantially lower liquid gradient than single-pass cross-flow trays. Accordingly, two-pass trays are particularly advantageous for contacting applications characterized by high liquid-gas ratios or large tray diameters.
Although the slotting arrangement taught by the aforementioned Matsch patent is capable of providing relatively efficient hydrualic behavior in application to single-pass cross-flow trays of moderate active area, coresponding advantage is not attained on the two-pass, side-to-center flow tray. The reason for this difference can be attributed to the structural configurational differences between the single-pass cross-flow tray and the two-pass, side-to-center flow design. In contrast to single-pass cross-flow trays, the divergent liquid inlet region of a side-to-center, two-pass tray is not contiguous with a converging outlet zone. It has been hypothesized that in a single-pass cross-flow tray provided with the Matsch improvement, the variable-directional slotting provided in the outlet zone provides a corrective influence over the inlet zone. Such corrective action and smoothing out of liquid gradient on the tray member contacting surface is not possible in the two-pass, side-to-center flow design, for the reason that there is no converging outlet section associated therewith.
Accordingly, it is an object of the present invention to provide an improved liquid-gas contact tray of the two-pass, side-to-center flow slotted sieve type, characterized by improved distribtuion of liquid across the entire active tray surface.
Another object of the invention is to provide improvement of the hydraulic behavior of large diameter cross-flow trays designed in accordance with the teachings of the Matsch patent.
Other objects and adavantages of this invention will be apparent from the ensuing disclosure and appended claims.